Protractor



April 13, 1943. F. H. PARKS 2,316,534

PROTRACTOR Filed Aug. l, 1941 Patented Apr. 13, 1943 UNITED vSTATES PATENT v'OFFICE PROTRACTOR Frank H. Parks, Portland, Oreg.

Application August 1, 1941, Serial No. 405,030

1 Claim.

This invention relates generally to navigation instruments and particularly to protractors.

The main object of this invention is to construct a protractor especially adapted for assisting a mariner in setting his course with the aid of a magnetic compass after the ships position has been determined by observation of two or more known charted landmarks or signals, or in any other manner such as by means of a radio beam. V

The second object is to construct a protractor which automatically sets olf the local magnetic variation once the protractor is set for this particular variation.

The third object is to construct a protractor of the class described whereby reckoning is reduced to a minimum and the time required for making a reckoning is relatively short, and the accuracy of the reckoning is increased materially due to the extreme simplicity of the operations involved.

These and other objects are accomplished in the manner set forth in the following specification as illustrated in the accompanying drawing, in which:

Fig. 1 is a plan of the device showing same in position on a map or chart.

Fig. 2 is an edge elevation of the device in which the thickness of the parts is magnified for the sake of clearness.

Similar numerals refer throughout the several views.

Referring in detail to the drawing, the device will be seen to consist of three principal parts. Upon the protractor base I are etched or printed the calibrations II indicating the degrees of a circle. The characters I2 for each tenth degree are indicated as well as the compass points I3.

The protractor base IIJ is preferably of a transparent material such as Celluloid or any of the well known plastic materials.

In the center of the base I0 is mounted a tubular rivet I4 upon which is rotatably mounted the magnetic course arm I5 preferably of colored or sanded material of reduced transparency. The magnet course arm I5 has the radial sides I6 which are in alinement with each other and occupy a plane passing through the axis of the rivet I4.

Also mounted on the rivet I4 is a transparent arm I'I whose edge I8 is also in alinement with the axis of the rivet I4. The arm Il may be of any convenient length, in actual practice being approximately eighteen inches long.

The arm I1 is provided with an arcuate slot to similar parts I9 through which extends a clamping screw 2i) whose nut 2l Ais'on the outside of the member II while its head 22 is recessed in the underside of the member I5. The purpose of the screw 2G is tomake it possible 4to clamp the members I5 and I'I togetherat an angle corresponding with the local variation. The arm Il is also preferably transparent although not necessarily so.

In order` to givea. clear understanding of the operationgof this device and the utility thereof, there is shown a portion of a marine chart 23 with suiicient topography to illustrate the purposes and advantages of the present invention.

On the chart is indicated the entrance 24 of the Columbia River with a jetty 25, a sand spit 26, McKenzie Head 21, the light house 28, the harbor lights 29 and 30, and a second sand spit 3| between which and the spit 26 is located the main channel 32. On the chart 23 are also indicated a parallel 33 and meridian 34. The chart also shows the variation 35 in degrees and minutes for a given year 36. In this particular case, the variation is twenty-two degrees and forty-five minutes east in the year 1939.

The operation of the device is as follows:

When navigating within the area covered by the chart 23, the rst operation to perform is to set the arm I5 with relation to the arm Il so as to make the angle between the edges I6 and I8 correspond with the variation 35 indicated on the chart 23, and to clamp same in this position by means of the nut 2 I.

Now, if it is desired to set a course, an observation is made when two known points or lights are in alinement. In the present illustration, the observation is made when the lights 28 and 29 are in alinement.

The observer then places the edge I8 of his instrument on the chart 23 intersecting the points 28 and 29 and then slides the base I along the chart 23 until one of the edges 2l or 38 are in register with a parallel 33 or meridian 34. Obviously he will find it necessary to turn the base Il) with relation to the arm II in order to accomplish this purpose.

Having oriented his instrument as explained, he may now wish to steer in the direction of the flashing red harbor light 30 which may at that moment be visible. He then turns the arm II to the position shown in dotted lines so that the edge I8 will intersect the harbor light 30 and the center of the rivet I4, and he finds that although the true course arm Il indicates a course of 56 and 45', his magnetic course is indicated at 34 east. That is, since the magnetic variation is 22,a

and 45 east of north in this locality, then his compass course must be that amount less than the course he wishes to navigate.

It is a well known fact that protractors have long been used for the purpose of plotting a ship's course from charts, examples of which are shown in the patent to Nash, 175,488. Such devices are based upon the assumption that the location of the vessel is known and that the designation point is visible.

With my device, it is only necessary to maneuver the vessel into alinement with two known points in order to determine the line on the chart along which the vessel lies, and the angularity of the course to be taken is measured on the chart, and the magnetic variation accounted-forand given to the navigator; the magnetic course to be followed by the vessel in orderto remain in the line of the lights. This course can be followed as long as desired.

If he wishes to change his course, he places his ship into alinement with two points which will permit him to determine the new magnetic reading which will be transmitted to the pilot house.

Obviously, the intersecting points of any two courses will give the precise location of theship itself. f

It will be noted that with my device, the true meridian pointer is not separatel from the compass chart, but is a part; thereof, and the base l0 having four sides 31 and 38 has the equivalent of four pointers in addition to the intersections between the zero and 180 mark and the 90 and 270 mark, thereby affording six possible lines for registering with meridians and parallels.

Attention is drawn to the fact that the magnetic course arm l5 is double ended. That is, it projects on opposite sides of the pivot I4. The purpose of this is to enable the mariner to determine the course in either direction.

I claim:

A protractor having a rectangular transparent base on the surface of which is formed a compass dial having a pivot disposed at the center thereof, a transparent true course arm mounted on said pivot and having one edge thereof in alinement thereto, said true course arm having an arcuate slot formed therethrough, the center of which is the center of the pivot, a magnetic l course arm also mounted on said pivot and having an end extending` across the face of said dial, both ends of said magnetic course arm being in alinenient with each other and radial from said pivot, and a clamping screw passing through the slot in said'true course arm into said magnetic course .arm whereby the` magnetic declination may be set oifl and held at a xed value while said 'true-andmagnetic course arms are permitted to rotate on said pivot.

FRANK H. PARKS. 

